This invention relates to improved bismuth-containing dielectric ceramic compositions suitable for producing ceramic capacitors. Mixed ion substitution and dopants are utilized in producing novel compositions that are stable over a wide temperature range.
Ceramics suitable for use in capacitors are characterized by a high dielectric constant. In conventional ceramics, however, the dielectric constant has shown a strong dependence on temperature. The behavior of the capacitance with respect to temperature is quantified in a single term called the Temperature Coefficient of Capacitance (hereafter referred to as TCC) which is expressed in terms of parts per million (ppm)/.degree. C. Careful material selection and processing have made possible the development of dielectric ceramic bodies that have very small TCC's over a wide range of temperature. This phenomenon can occur when a multiphase body is formed in which each phase with a positive TCC is counterbalanced by one or more phases with an equivalently negative TCC. Certain bodies in which this compensating effect is present have Temperature Coefficients of Capacitance that are unusually small and are known as Negative-Positive-Zero or NPO bodies.
Though the acronym NPO denotes zero variation in the capacitance, the limit of acceptability for capacitors, as dictated to industry by the Electronic Industries Association (EIA), is .+-.30 ppm/.degree. C. from room temperature across the temperature range of -55.degree. C. to 125.degree. C. at a frequency of either 1 KHz or 1 MHz, depending on the capacitance of the material. By the same standards, the loss tangent must not be greater than 0.1% at the same frequencies and at room temperature.
Temperature compensation has typically been approached in one of two ways: first, the mixtures of two types of compounds with an opposite sign of TCC have been chosen such that the positive TCC of one type of compound(s) compensates the negative TCC of the other type of compound(s); or second, mixtures of doped or undoped layers, such as SrTiO.sub.3, and a layered structure compound have been chosen such that the positive TCC of the layer compensates the negative TCC of the layered structure compound. By varying the relative proportions of the compounds used, a series of temperature compensating capacitors with various values of TCC can be obtained.
The concept of temperature compensating ceramic capacitors is not new to the scientific community. An NPO body utilizing varying ratios of MgTiO.sub.3 and TiO.sub.2 was developed as early of the 1930's; it had a dielectric constant of 15. At the same time exploration of the SrO-TiO.sub.2 system yielded an NPO body with a dielectric constant of 25. The ensuing two decades hosted much research on NPO materials, being highlighted by the exploration of the BaO-4.6TiO.sub.2 system which yielded an NPO body with a dielectric constant of 35. This research also led to the exploration of the rare earth oxide-titania system and the development of an NPO body with a dielectric constant of 62 in the 1960's. Currently, most of the developmental work on NPO-type dielectrics is based on the rare earth oxide-titania system, where materials with dielectric constants of 80 and 102 have been developed. These materials are the basis of ceramic capacitor manufacturing today.
Ceramic capacitors have proven to be an integral part of the electronics industry where the increased emphasis on volume efficiency has catalyzed an effort to not only find novel materials, but also to make smaller and smaller capacitors to keep pace with the continuing volume reduction of the silicon chip. Temperature stable ceramic capacitors have proven useful in areas where a constant voltage is needed or diversion of potentially damaging electrical surges is desired. These include timing circuits, television tuner circuits, resonator circuits, and voltage multiplying rectifiers of oscilloscopes.